FIG. 1 shows a typical topological structure of a three-phase power converter comprising: a power converter unit (or referred to as a semiconductor switching unit) 12 which comprises: three bridge arms each being comprised of two groups of semiconductor switching elements connected in series respectively, midpoints A, B and C of the three bridge arms acting as AC terminals, and DC terminals O, P and Q formed by DC bus filter capacitors CB0, CB1 and CB2, wherein the terminal O is a common terminal of the filter capacitors CB1 and CB2; a filter inductance group 14′ having one set of ends connected to a grid 11 and the other set of ends connected to the midpoints A, B and C of the bridge arms of the semiconductor switching unit 12; a filter capacitor group 13 comprising three star-connected filter capacitors Cx each having one end connected to the grid 11 and the other end connected with other capacitors Cx to form a neutral point N. The filter inductance group 14′ and the filter capacitor group 13 constitute a differential mode filter.
The grid 11 may be referred to as an AC voltage unit, for example, it may be an AC voltage of about 50 Hz, such as a three-phase AC power supply (for supplying power to equipments) on a grid side or a power receiving unit (for receiving electrical energy generated by a generator) on a motor side.
Moreover, the power converter may further comprise a Line Impedance Stabilization Network (LISN) 16 which is an auxiliary equipment for testing conducted electromagnetic interference.
Provided that there is a distributed (or so-called parasitic) capacitor C0 between a DC bus connected to the DC bus filter capacitors CB0, CB1 or CB2 and the ground, and C1A, C1B and C1C are distributed capacitors respectively between the midpoints of each of the bridge arms of the semiconductor switching unit and the ground. Potential jumping of the midpoints of the DC bus relative to the ground, as well as potential jumping of the midpoints of the bridge arms relative to the ground, may cause displacement currents through the distributed capacitors, and the displacement currents flowing to the ground may cause common mode noise. In order to meet the international EMC standards, it is a common concern in the field to suppress common mode noise effectively with low cost.
FIG. 2 illustratively shows a conventional solution for suppressing common mode noise using an inactive common mode filter 17 to suppress common mode noise. The common mode filter 17 comprises a common mode inductance 171 and a filter capacitor group 172 formed by three star-connected filter capacitors Cy. The common mode inductances usually are big in volume and high in cost. When the common mode inductances are required to have high common mode inductance value, it is difficult to design the common mode inductances.
Another kind of conventional solution depresses the requirements on the common mode filters by reducing original common mode noise. As shown in FIG. 3, the neutral point N of the filter capacitor group 13 is connected to the midpoint O of the DC bus. Since N is a virtual neutral point whose potential is relatively stable, the potential of the bus relative to the ground is also clamped to a stable potential after the DC bus midpoint O is connected to the virtual neutral point N, thus it can reduce common mode noise to some extent. A variation of this solution is, as shown in FIG. 4, considering that in direct connection a current of zero sequence component on the connection line may be high, a capacitor may be connected into the connection line between the neutral point N of the filter capacitor and the midpoints O of the DC bus, such that the current of zero sequence component may be controlled to a proper value. However, this kind of solution can only suppress the common mode noise flowing to the ground through the distributed capacitor C0 between the DC bus and the ground, and the common mode noise caused by the distributed capacitors C1A, C1B and C1C between the midpoints of the bridge arms and the ground are not able to be suppressed, but instead, to be increased.